It is, according to Mr Grass, the largest underground chamber so far discovered in the Mendip Hills.

"There are two floor-to-ceiling columns or pillars both five metres long and pure white," said Mr Grass.

"And it's filled with stalactites and stalagmites which are some of the best discovered not just on the Mendips but in the UK.

"It's only 20 miles from Bristol but other than the six of us, nobody has ever been there - so it's quite a thing."

Graham Price, chairman of the Council of Southern Caving Clubs, said the previous largest cave found in the Mendips had been GB Cavern, a 6m (19ft) wide by 12m (39ft) high by 90m (295ft) long chamber discovered in the 1940s.

"You just don't find things of this size in the Mendips - chambers that you can walk through," he said.

"So we're really thrilled for the group and really, really excited about it."